The invention relates generally to a process and apparatus for the combined grinding and drying (referred to herein as grind-drying) of wet or moist solid fuels to form dust, or finely grained particles, in a fan type mill. Fuels to be processed by the process and apparatus include brown coal or lignite, which may be in coarsely crushed form.
In this specification, the terms brown coal and lignite may be used interchangeably. In other words, a reference to brown coal is intended to include lignite and a reference to lignite is intended to include brown coal. Although brown coal and lignite are often generally considered as the same recent knowledge indicates there may be a difference between the two types of coal. Notwithstanding any such difference, both are encompassed by the present invention. For convenience and brevity, the term brown coal is generally used herein to denote both kinds of material.
One method for the economic production of large amounts of fuel as finely grained particles, or dust form, as is required for example for refining or upgrading brown coal (or lignite) in gasification and liquefaction installations or in firing equipment, is grind-drying as is predominantly used in modern furnaces for steam generators which are fired with brown coal in power generating stations. Such a method involves using a mill which is often referred to as a fan type mill (beater wheel mill) comprising for example a fan and beater arms secured to the fan. The mill thus sucks in a hot drying gas, which more particularly may have a low oxygen content, as is required for drying the crude or as-mined brown coal, together with the brown coal, crushes the coal by means of the beater arms secured to the fan blades, and then discharges the entire gas flow, together with the brown coal dust produced, from the mill.
When using the above-outlined grind-drying process in mills for power generating stations, flue gas or boiler gas is drawn at a temperature of about 1000.degree. C. from the firing chamber and conducted to the mill in a drying section which is generally in the form of a gravity feed shaft. At a suitable distance upstream of the mill, the crude coal which is to be processed is introduced into the gravity feed shaft, so that it is then sucked in by the fan mill, together with the flue gas; the coal now begins to undergo drying in the flow of gas in the shaft. Depending on the amount of coal introduced into the gas flow and the moisture content thereof, hot air, cold air or other cold gas is mixed with the flue gas; the respective proportion of hot air, cold air or cold gas in the drying gas depending on the temperatures in a sifter or separator downstream of the mill. At the point at which the coal is introduced into the gravity feed shaft, with the coal usually being supplied to the gravity feed shaft upstream of the mill in a grain size range of from 0 to 80 mm, the flue gas temperatures are between 600.degree. C. and 800.degree. C. The effect of the hot drying gas on the coal which is added thereto is to cause spontaneous vaporization of a part of the surface water on the coal, in which case the temperature of the gas in the gravity feed shaft abruptly drops. The larger pieces of coal are partly broken up due to the vaporization phenomenon. However, it will be appreciated that the actual step of grinding or crushing the coal is then carried out in the mill. The dust-charged mixture of coal, flue gas, air and vapor (steam) which issues from the mill is first sifted or separated once again, in order to separate out any excessively large pieces of coal, and then injected in the usual manner through burners into the boiler of the steam generator. With the above-described mode of operation, it does not matter whether volatile constituents are expelled from the coal at the stage of the drying process, as the entire mixture is in any case fed directly to the boiler (except of course for the above-mentioned large lumps of coal which are separated out).
The operation of adding hot air, cold air or cold gas to the flue or boiler gas for the purposes of adjusting the drying gas to given, predetermined values is known as conditioning. However, when the grinding installation is partly supplied with crude coal or when the coal has a low water content, it may also be necessary for water additionally to be introduced into the mill. The additional water is usually added by way of nozzles. The addition of water simulates a higher moisture content in the coal. However, a higher moisture content in the coal requires a larger amount of drying gas; the vapors which are produced in larger quantities as a result of the addition of water promote inerting of the process conditions, to which further reference will be made hereinafter.
The grind-drying installations for carrying out the above-indicated steps are simple in design and reliable in operation, and are accordingly economical. However, while they produce a high proportion of fine discharge product, it is still mixed with comparatively coarse material so that the ground material has to be subsequently sifted or graded, if the aim of the process is to be exclusively on production of dust or finely grained particles. A further requirement in regard to the production of brown coal (or lignite) dust is concerned with rendering inert the conditions under which grinding is effected, as brown coal dust is known to ignite easily. In this connection, it is generally necessary for the O.sub.2 -content of the drying gas to be lower than 8%.
The effect to which reference has already been made above, of driving out of the coal volatile constituents, which are often referred to as the valuable or useful substances, necessarily results in the caloric value of the remaining coal being reduced, which is a disadvantage at any rate when the coal is not subsequently burnt in conjunction with the volatile constituents which have been previously expelled therefrom. In addition, there is always a possibility of the expelled volatile constituents resulting in inadmissible emissions or giving rise to the danger of an explosion.
When drying brown coal, volatile constituents are driven out at relatively low temperatures. Thus, for example, CO.sub.2 is driven out when the temperature reaches about 150.degree. C. to 180.degree. C., while methane (CH.sub.4) is driven out of the coal from about 250.degree. C. From a temperature of about 300.degree. C. to 350.degree. C., the oxygen present in the brown coal is liberated and passes into the atmosphere in the grinding chamber, and that is a disadvantage from the point of view of carrying out the process under inert conditions, as is required. That same point also arises, although possibly to a lower degree, in regard to fuels other than brown coal.